U.S. patent number 7,535,609 [Application Number 11/339,700] was granted by the patent office on 2009-05-19 for hologram recording method and device.
This patent grant is currently assigned to Fuji Xerox Co., Ltd.. Invention is credited to Makoto Furuki, Koichi Haga, Kazuhiro Hayashi, Katsunori Kawano, Jiro Minabe, Yasuhiro Ogasawara, Shin Yasuda, Hisae Yoshizawa.
United States Patent |
7,535,609 |
Yasuda , et al. |
May 19, 2009 |
Hologram recording method and device
Abstract
A hologram recording method comprising recording information of
signal light as a reflection-type hologram on an optical recording
medium, by illuminating the signal light and reference light on a
same axis and from different sides of the optical recording medium
is provided. Further, a hologram recording device including: a
signal light illuminating section illuminating signal light onto an
optical recording medium; and a reference light illuminating
section illuminating reference light of a same axis as the signal
light, onto the optical recording medium from a different side than
the signal light is provided.
Inventors: |
Yasuda; Shin (Ashigarakami-gun,
JP), Kawano; Katsunori (Ashigarakami-gun,
JP), Haga; Koichi (Ashigarakami-gun, JP),
Minabe; Jiro (Ashigarakami-gun, JP), Furuki;
Makoto (Ashigarakami-gun, JP), Ogasawara;
Yasuhiro (Ashigarakami-gun, JP), Hayashi;
Kazuhiro (Ashigarakami-gun, JP), Yoshizawa; Hisae
(Ashigarakami-gun, JP) |
Assignee: |
Fuji Xerox Co., Ltd. (Tokyo,
JP)
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Family
ID: |
37767090 |
Appl.
No.: |
11/339,700 |
Filed: |
January 26, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070041066 A1 |
Feb 22, 2007 |
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Foreign Application Priority Data
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Aug 19, 2005 [JP] |
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2005-238823 |
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Current U.S.
Class: |
359/29; 359/22;
369/103 |
Current CPC
Class: |
G03H
1/04 (20130101); G11B 7/0065 (20130101); G11B
7/083 (20130101); G03H 1/0404 (20130101); G03H
1/12 (20130101); G03H 2001/0415 (20130101) |
Current International
Class: |
G03H
1/16 (20060101); G03H 1/26 (20060101); G11B
7/00 (20060101) |
Field of
Search: |
;359/22,24,25,29
;369/103 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kazuhiko Kimura, "Improvement of the optical signal-to-noise ration
in common-path holographic storage by use of a
polarization-controlling media structure," Optics Letter, vol. 30,
No. 8, Apr. 15, 2005, pp. 878-880. cited by other.
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Primary Examiner: Amari; Alessandro
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A hologram recording/reconstructing method for recording
information of signal light as a reflection-type hologram on an
optical recording medium and reconstructing the recorded
information, the method comprising: emitting a laser light;
generating, from the emitted laser light, a signal light and a
reference light during the time of recording the information, and
the reference light during the time of reconstructing the recorded
information, the signal light and the reference light being
linearly polarized lights; providing a half-wave plate to change
the electric field oscillation direction of a light to make the
electric field oscillation directions of polarization the same for
the signal light and the reference light during the time of
recording the information; and illuminating, during the time of
recording the information, the signal light and the reference light
as a recording light on a same axis from opposite surface sides of
the optical recording medium, and illuminating, during the time of
reconstructing the recorded information, the reference light as a
reading light, wherein the method further comprises positioning a
single quarter wave plate on an optical path of the reference light
during the time of reconstructing the recorded information, and
positioning the single quarter wave plate away from the optical
path during the time of recording the information.
2. The hologram recording/reconstructing method of claim 1, further
comprising generating the signal light by modulating reference
light which has been transmitted through the optical recording
medium.
3. The hologram recording/reconstructing method of claim 2, further
comprising modulating, by a spatial light modulator, the reference
light which has been transmitted through the optical recording
medium.
4. The hologram recording/reconstructing method of claim 3, wherein
the spatial light modulator is a reflection-type spatial light
modulator.
5. The hologram recording/reconstructing method of claim 4, further
comprising: generating signal light by modulating, by the spatial
light modulator, the reference light which has been transmitted
through the optical recording medium; reflecting the generated
signal light at an optical recording medium side; and illuminating
the signal light and reference light on a same axis and from
different sides of the optical recording medium.
6. The hologram recording/reconstructing method of claim 2, further
comprising generating the signal light by modulating zeroth-order
light included in the reference light which has been transmitted
through the optical recording medium.
7. The hologram recording/reconstructing method of claim 2, further
comprising illuminating, onto the optical recording medium,
reference light whose intensity distribution or phase distribution
has been modulated.
8. The hologram recording/reconstructing method of claim 7, wherein
the reference light, whose intensity distribution or phase
distribution has been modulated, is generated such that an
intensity ratio of the signal light and the reference light is a
predetermined value.
9. The hologram recording/reconstructing method of claim 7, wherein
the reference light is generated by using a computer generated
hologram.
10. The hologram recording/reconstructing method of claim 2,
further comprising further modulating an intensity distribution or
a phase distribution of the reference light such that an intensity
ratio of the signal light and the reference light is a
predetermined value, and illuminating the signal light and the
reference light onto the optical recording medium.
11. The hologram recording/reconstructing method of claim 2,
wherein, at a Fourier transform plane of the signal light, a region
of first-order diffracted light due to a maximum spatial frequency
included in a reference light pattern, encompasses a region of
first-order diffracted light due to a maximum spatial frequency
included in a signal light pattern.
12. The hologram recording/reconstructing method of claim 2,
wherein, at a Fourier transform plane of the signal light, a region
of first-order diffracted light due to a maximum spatial frequency
included in a signal light pattern, and a region of first-order
diffracted light due to a maximum spatial frequency included in a
reference light pattern, are equal.
13. The hologram recording/reconstructing method of claim 1,
further comprising disposing the optical recording medium at a
Fourier transform plane of the signal light.
14. The hologram recording/reconstructing method of claim 1,
further comprising disposing the optical recording medium at a
position which is offset, in an optical axis direction, from a
Fourier transform plane of the reference light.
15. The hologram recording/reconstructing method of claim 1,
further comprising, in a case in which a position of a
light-collecting point of the signal light and a position of a
light-collecting point of the reference light are different,
disposing the optical recording medium at a position other than the
position of the light-collecting point of the signal light and the
position of the light-collecting point of the reference light.
16. The hologram recording/reconstructing method of claim 1, the
method further comprising: emitting a reconstructing laser light
for reconstructing, which is linearly polarized; transmitting the
reconstructing laser light through a polarization beam splitter;
illuminating the reconstructing laser light through a quarter-wave
plate onto the optical recording medium to obtain a diffracted
light; transmitting the diffracted light through the quarter-wave
plate; reflecting the diffracted light by the polarization beam
splitter; and detecting the reflected light by a detector.
17. A hologram recording/reconstructing device comprising: a laser
light source emitting a laser light; a signal light generating
section that generates a signal light which is linearly polarized;
a reference light generating section that generates a reference
light which is linearly polarized, wherein the electric field
oscillation directions of polarization are the same for the signal
light and the reference light when the device is recording the
information; a signal light illuminating section that illuminates
the signal light onto an optical recording medium when the device
is recording the information; a reference light illuminating
section that illuminates the reference light of a same axis as the
signal light, onto the optical recording medium from a different
side than the signal light when the device is recording the
information and reconstructing the recorded information; and a
single quarter wave plate being positioned on an optical path of
the reference light when the device is reconstructing the recorded
information, and being positioned away from the optical path when
the device is recording.
18. The hologram recording/reconstructing device of claim 17,
wherein: the reference light illuminating section is structured by
an illuminating optical system which illuminates the laser light,
which is emitted from the laser light source, from one surface side
of the optical recording medium; and the signal light illuminating
section is structured by a spatial light modulator, which generates
signal light by spatially modulating reference light transmitted
through the optical recording medium, and an illuminating optical
system, which illuminates the signal light, which is generated by
the spatial light modulator, on a same axis as the reference light
and from another surface side of the optical recording medium.
19. The hologram recording/reconstructing device of claim 18,
wherein the illuminating optical system of the signal light
illuminating section includes a first lens whose focal length is fs
and which collects the signal light on the optical recording
medium, and the illuminating optical system of the reference light
illuminating section includes a second lens whose focal length is
ft and which collects the reference light on the optical recording
medium, and the first lens and the second lens are disposed such
that a distance between the first lens and the second lens is
different than a sum (fs+fr) of the focal lengths of the both
lenses.
20. The hologram recording/reconstructing device of claim 17,
wherein the illuminating optical system of the signal light
illuminating section includes a first lens whose focal length is fs
and which collects the signal light on the optical recording
medium, and the illuminating optical system of the reference light
illuminating section includes a second lens whose focal length is
fr and which collects the reference light on the optical recording
medium, and the first lens and the second lens are disposed such
that a distance between the first lens and the second lens is
different than a sum (fs+fr) of the focal lengths of the both
lenses.
21. The hologram recording/reconstructing device of claim 17,
further comprising: a shutter selectively emitting a reconstructing
laser light for reconstructing, which is linearly polarized; a
polarization beam splitter and a quarter-wave plate provided, in
this order from the light source to the optical recording medium,
on an optical path, which is common to both of an optical path of
the reconstructing laser light from the light source to the optical
recording medium and an optical path of an diffracted light
diffracted by the optical recording medium, the polarization beam
splitter transmitting the reconstructing laser light emitted by the
light source and reflecting the diffracted light diffracted by the
optical recording medium; and a detector detecting the diffracted
light reflected by the polarization beam splitter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority under 35 USC 119 from Japanese
Patent Application No. 2005-238823, the disclosure of which is
incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hologram recording method and
device, and in particular, to a hologram recording method and
device which record a reflection-type hologram by illuminating
signal light and reference light on the same axis.
2. Description of the Related Art
A method of fabricating a transmission-type hologram by
illuminating signal light and reference light on the same axis from
the same surface side of a recording medium has been proposed (see
Japanese Patent No. 3452113). In this method, reference light and
signal light generated from spatially different positions of a
spatial light modulator are Fourier transformed by a lens. Because
the Fourier transformed signal light and reference light are
superposed in a vicinity of the Fourier transform plane, by placing
the recording medium at this position, a hologram can be recorded.
Further, in this method, because the signal light and the reference
light are illuminated on the same axis, the optical system is
simple, and the recording device can be made to be compact.
However, the further away, in the direction of the optical axis,
from the Fourier transform plane, the smaller the region at which
the signal light and the reference light are superposed.
Accordingly, in same-axis recording of a transmission-type
hologram, there is the major problem that, if the film thickness of
the recording material is large, a hologram cannot be recorded in
the entire optical axis direction (direction of thickness) of the
recording medium.
In a volume hologram in which interference fringes are recorded
three-dimensionally by utilizing the direction of thickness of the
recording medium, the greater the thickness of the material, the
stricter the Bragg condition, and the larger the dynamic range can
be made. Further, the stricter the Bragg condition and the larger
the dynamic range, the greater the number of holograms that can be
multiple-recorded. Accordingly, the greater the thickness of the
material, the greater the number of holograms that can be
multiple-recorded.
Moreover, in order to make the recording density large, the
recording region must be made to be small. In order to make the
recording region small, it is desirable to make the focal length of
the Fourier transform lens short. However, it is difficult to both
make the thickness of the recording medium thick and make the focal
length of the Fourier transform lens short. The reason for this is
as follows: the recording medium exists at a position which is
separated, in the optical axis direction, from the Fourier
transform plane, and the shorter the focal length of the Fourier
transform lens, the smaller the region at which the signal light
and the reference light are superposed, even at a position which is
slightly away from the Fourier transform plane in the optical axis
direction. Therefore, the recording of a hologram is difficult at
this position.
In order to realize a high recording density in this way, a thick
recording material and a Fourier transform lens having a short
focal length are needed. However, in same-axis recording of a
transmission-type hologram, it is difficult to both make the
thickness of the recording medium thick and make the focal length
of the Fourier transform lens short, and therefore, it is difficult
to realize high density recording.
Further, the aforementioned prior art proposes providing a
reflective layer at the reverse surface of the recording medium.
However, in this case, the quality of the signal light and the
reference light is affected greatly by the quality of the
reflective layer, and the problem arises that the quality of the
signal light and the reference light deteriorates due to defects of
the reflective layer or the like. Moreover, there is the problem
that, in the reconstructing of data as well, it is easy for the SN
ratio of the reconstruction light to deteriorate. The reason for
this is that, because the reading light and the reconstruction
light exit in the same direction on the same axis, scattering light
of the reading light is incident on a detector together with the
reconstruction light.
As a method of reducing this scattering light, Optics Letters Vol.
30 p 878-880 (2005) proposes sandwiching a recording material by
quarter-wave plates. In this method, a reflection-type hologram is
recorded by incident light of signal light and reflected light from
the reflective layer of the reference light, and reflected light
from the reflective layer of the signal light and incident light of
the reference light. At the time of reconstructing, because the
reconstruction light and the reading light are linearly polarized
lights whose planes of polarization are orthogonal, the scattering
light can be reduced by using an analyzer.
However, in this method, because quarter-wave plates are used at
the recording medium, there are the problems that the recording
medium is expensive and the production thereof also is complex.
Moreover, when this recording medium is rotated around the optical
axis, the optical axes of the quarter-wave plates become offset,
and therefore, there is the problem that the recording medium
cannot be used as a rotating-type medium such as a DVD.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above
circumstances and provides a hologram recording method and device
which can record a reflection-type hologram by illuminating signal
light and reference light on the same axis, and which enable high
density recording at a volume hologram. Further, the present
invention provides a hologram recording method and device in which
an optical system can be simplified and a recording device can be
made compact.
A first aspect of the present invention is a hologram recording
method including: recording information of signal light as a
reflection-type hologram on an optical recording medium, by
illuminating the signal light and reference light on a same axis
and from different sides of the optical recording medium.
Further, a second aspect of the present invention is a hologram
recording device including: a signal light illuminating section
illuminating signal light onto an optical recording medium; and a
reference light illuminating section illuminating reference light
of a same axis as the signal light, onto the optical recording
medium from a different side than the signal light.
In order to illuminate the signal light and the reference light on
the same axis and from different sides of the optical recording
medium, it is particularly preferable to modulate the reference
light, which has been transmitted through the optical recording
medium, and generate the signal light. For example, the reference
light, which has been transmitted through the optical recording
medium, can be modulated by a reflection-type spatial light
modulator so as to generate the signal light, the generated signal
light can be reflected at the optical recording medium side, and
the signal light and the reference light can be illuminated on the
same axis from different sides of the optical recording medium.
As described above, the present invention has the effects that a
reflection-type hologram can be recorded by illuminating signal
light and reference light on a same axis, and high-density
recording at a volume hologram is possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will be described in
detail based on the following figures, wherein:
FIG. 1 is schematic structural diagram of a hologram
recording/reconstructing device relating to a first embodiment of
the present invention;
FIG. 2 is a diagram showing the relationship between focal point
positions of lenses and a position of a hologram recording
medium;
FIG. 3 is a diagram showing the relationship between focal lengths
of lenses and a distance between the lenses;
FIG. 4 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a second embodiment of
the present invention;
FIG. 5 is a diagram showing directions of moving a hologram
recording medium;
FIG. 6 is a diagram showing a modified example in which a long
focal length lens is inserted;
FIG. 7 is a diagram showing a modified example in which a
diffraction element is inserted;
FIG. 8 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a third embodiment of
the present invention;
FIG. 9A is a front view showing the structure of a diffraction
element used in the third embodiment;
FIG. 9B is a cross-sectional view along arrow A-A of FIG. 9A;
FIG. 10 is a diagram showing a state in which only light, which has
passed through a hollow region of the diffraction element, is
incident on a spatial light modulator; and
FIG. 11 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a fourth embodiment of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Examples of embodiments of the present invention will be described
in detail hereinafter with reference to the drawings.
First Embodiment
FIG. 1 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a first embodiment of
the present invention. As shown in FIG. 1, a laser oscillator 10
using, for example, an Nd:YVO.sub.4 crystal, is provided at the
hologram recording/reconstructing device. Laser light which is
coherent light of, for example, a wavelength of 532 nm, is
oscillated and illuminated from the laser oscillator 10.
A shutter 12, which is for blocking laser light, is disposed at the
laser light illuminating side of the laser oscillator 10, so as to
be able to withdraw from the optical path. A half-wave plate 14
which rotates the plane of polarization and adjusts the intensity
ratio of signal light and reference light, a pair of enlarging
lenses 16, 18 which collimate laser light into a beam of a large
diameter, a polarization beam splitter 22 which, by transmitting
P-polarized light and reflecting S-polarized light, separates the
laser light into two types of light which are light for reference
light and light for signal light, and a reflecting mirror 20 which
reflects the laser light and changes the optical path to the
direction of the polarization beam splitter 22, are disposed at the
light transmitting side of the shutter 12.
Reflecting mirrors 24, 28, which reflect the P-polarized light,
which is transmitted through the polarization beam splitter 22, and
change the optical path to the direction of a hologram recording
medium 38, are provided at the light transmitting side of the
polarization beam splitter 22. A shutter 26, which is for blocking
the laser light for signal light, is disposed between the
reflecting mirror 24 and the reflecting mirror 28, so as to be able
to withdraw from the optical path.
A half-wave plate 30 which rotates the plane of polarization of the
reflected P-polarized light and adjusts the contrast of the signal
light, a transmission-type spatial light modulator 32 which is
structured by a liquid crystal display element or the like and
which modulates the laser light for signal light in accordance with
a supplied recording signal of each page and which generates the
signal light (P-polarized light) for recording each page of the
hologram, and a lens 34 which collects the laser light for signal
light, are disposed in that order at the light reflecting side of
the reflecting mirror 28. The lens 34 illuminates the P-polarized
light as signal light onto the hologram recording medium 38. The
optical axis of the half-wave plate 30 is adjusted so that, for
example, the contrast of the signal light transmitted through the
spatial light modulator 32 becomes a maximum.
A light-shielding plate 36, in which an aperture 36A is formed, is
disposed next to the hologram recording medium 38 at the signal
light incident side of the hologram recording medium 38. The signal
light collected at the lens 34 passes through the aperture 36A
formed in the light-shielding plate 36 and is illuminated onto the
hologram recording medium 38.
A half-wave plate 40 which rotates by 90.degree. the plane of
polarization of the S-polarized light reflected at the polarization
beam splitter 22, a reflecting mirror 42 which reflects the
P-polarized light and changes the optical path to the direction of
the hologram recording medium 38, and a polarization beam splitter
44 which transmits the reflected laser light (P-polarized light),
are disposed at the light reflecting side of the polarization beam
splitter 22.
A quarter-wave plate 46, which converts linearly polarized light
into circularly polarized light or circularly polarized light into
linearly polarized light, is disposed at the light transmitting
side of the polarization beam splitter 44, so as to be able to
withdraw from the optical path. The quarter-wave plate 46 is
withdrawn from the optical path during recording, and is inserted
on the optical path during reconstructing. A lens 48, which
collects laser light for reference light and generates reference
light which is formed from a spherical reference wave, is disposed
at the light transmitting side of the quarter-wave plate 46. The
lens 48 illuminates P-polarized light as reference light onto the
hologram recording medium 38. The signal light and the reference
light are thereby illuminated simultaneously onto the hologram
recording medium 38. At this time, the reference light is collected
such that the optical axis thereof is coaxial with the optical axis
of the signal light, and the reference light is illuminated onto
the hologram recording medium 38 from a different side than the
signal light.
Lenses 50, 52, and a detector 54, which is structured by an image
pickup element such as a CCD or the like and which converts
received reconstruction light into an electric signal and outputs
the electric signal, are disposed at the light reflecting side of
the polarization beam splitter 44 at the reconstruction light
reconstructed from the recording medium 38. The detector 54 is
connected to a personal computer (not shown). The personal computer
is connected to the spatial light modulator 32 via a pattern
generator which generates a pattern in accordance with a recording
signal supplied at a predetermined timing. Further, a driving
device (not shown) is connected to the personal computer. The
driving device drives the shutters 12, 26 and the quarter-wave
plate 46 so as to respectively insert them individually into the
optical path, and causes the shutters 12, 26 and the quarter-wave
plate 46, which have been inserted in the optical path, to be
individually withdrawn from the optical path.
FIG. 2 is a drawing showing the relationship between the focal
point positions of the lenses and the position of the hologram
recording medium. As shown in FIG. 2, given that the focal length
of the lens 34 is f.sub.1 and the focal length of the lens 48 is
f.sub.2, the distance between the lens 34 and the lens 48 is the
sum of their focal lengths, i.e., f.sub.1+f.sub.2. At this time, it
is preferable that the hologram recording medium 38 be disposed at
a position which is offset along the optical axis direction from
the Fourier transform plane (hereinafter called "FT plane"), i.e.,
the common focal point position of the two lenses. If the hologram
recording medium 38 is disposed at the FT plane, the signal light
and the reference light can only be superposed at the focal point,
and therefore, the necessary information of the signal light cannot
be recorded. In contrast, if the hologram recording medium 38 is
offset from the FT plane, the superposition of the signal light and
the reference light becomes large, and the information of the
signal light can be recorded without drop-out.
The hologram recording processing will be described next. First,
the driving device (not shown) is driven, and the shutters 12, 26
and the quarter-wave plate 46 are respectively withdrawn from the
optical path such that the laser light can pass through. Next,
laser light is illuminated from the laser oscillator 10, a
recording signal for each page is supplied from the personal
computer (not shown) to the spatial light modulator 32 at a
predetermined timing, and hologram recording processing onto the
hologram recording medium 38 is carried out.
Namely, the plane of polarization of the laser light exiting from
the laser oscillator 10 is rotated by the half-wave plate 14, and
the laser light is collimated into a large-diameter beam at the
enlarging lenses 16, 18 and is incident on the reflecting mirror
20. The laser light reflected at the reflecting mirror 20 is
incident on the polarization beam splitter 22. The laser light is
separated by the polarization beam splitter 22 into two types of
light which are light (S-polarized light) for reference light and
light (P-polarized light) for signal light.
The P-polarized light which is transmitted through the polarization
beam splitter 22 is reflected at the reflecting mirrors 24, 28, the
plane of polarization thereof is rotated at the half-wave plate 30,
and the light is modulated at the spatial light modulator 32 in
accordance with the recording signal such that signal light is
generated. The generated signal light of P-polarized light is
collected at the lens 34, passes through the aperture 36A formed in
the light-shielding plate 36, and is illuminated onto the hologram
recording medium 38. In order to achieve both a high recording
density and a high SN ratio of the reconstructed image, given that
the focal length of the lens 34 is f.sub.1, the focal length of the
lens 48 is f.sub.2, the pixel interval of the spatial light
modulator 32 is p.sub.1, and the wavelength of the laser light is
.lamda., the configuration of the aperture 36A is made to be
square, and the length of one side thereof is preferably in a range
of 1.lamda.f.sub.1/p.sub.1 to 2.5.lamda.f.sub.1/p.sub.1, and is
more preferably in a range of 1.lamda.f.sub.1/p.sub.1 to
1.5.lamda.f.sub.1/p.sub.1. The central position of the aperture 36A
coincides with the optical axis.
On the other hand, the plane of polarization of the S-polarized
light, which is reflected at the polarization beam splitter 22, is
rotated by 90.degree. at the half-wave plate 40, and the light is
reflected at the reflecting mirror 42 and incident on the
polarization beam splitter 44. The P-polarized light which is
transmitted through the polarization beam splitter 44 is collected
at the lens 48, and reference light which is formed from a
spherical reference wave is generated. The generated reference
light of P-polarized light is illuminated onto the hologram
recording medium 38 on the same axis as and from a different side
than the signal light.
In this way, the signal light and the reference light are
simultaneously illuminated onto the hologram recording medium 38.
Changes in the refractive index or the absorption thereby arise due
to the interference between the signal light and the reference
light at the places where the lights strengthen one another,
whereas there is little of such changes at the places where the
lights weaken one another. Hologram recording of each page is
carried out by this phenomenon. Here, due to the signal light and
the reference light being illuminated from different sides of the
hologram recording medium 38, a reflection-type hologram, at which
high density recording in the thickness direction is possible, is
recorded in the hologram recording medium 38.
The hologram reconstructing processing will be described next.
First, the driving device (not shown) is driven, and the shutter 26
and the quarter-wave plate 46 are respectively inserted onto the
optical path. The laser light transmitted through the polarization
beam splitter 22 is thereby cut-off at the shutter 26, and
therefore, only the reference light is illuminated onto the
hologram recording medium 38 in which a hologram is recorded.
Further, the P-polarized light which is transmitted through the
polarization beam splitter 44 is converted into circularly
polarized light at the quarter-wave plate 46, is collected at the
lens 48, and is illuminated onto the hologram recording medium
38.
The reconstruction light which is diffracted at the hologram
recording medium 38 becomes reverse-direction circularly polarized
light, and exits at the lens 48 side. The reverse-direction
circularly polarized light is transmitted through the lens 48, is
converted into linearly polarized light (S-polarized light) at the
quarter-wave plate 46, and is incident on the polarization beam
splitter 44. Only the S-polarized light which is the reconstruction
light is selectively reflected at the polarization beam splitter
44, and is received at the detector 54 via the lenses 50, 52. The
reconstruction light received at the detector 54 is converted into
an electric signal and inputted to the personal computer (not
shown), and is displayed on a display (not shown) provided at the
personal computer. In this way, the hologram image of each page is
reconstructed.
As described above, in the first embodiment, because the signal
light and the reference light are illuminated onto the hologram
recording medium on the same axis and from opposite directions, a
reflection-type hologram can be recorded. In this reflection-type
hologram, even if the hologram recording medium is disposed at a
position which is offset in the optical axis direction from the
Fourier transform plane, it is possible to make the superposition
of the signal light and the reference light large, the hologram can
be multiple-recorded by utilizing the direction of thickness of the
recording medium, and high density recording can be achieved.
Further, the recording/reconstructing of a reflection-type hologram
has advantages such as: (1) there is no need to provide a
reflective layer at the hologram recording medium, and
deterioration of the signal light and the reference light can be
prevented; (2) because the propagating directions of the reference
light and the reconstruction light illuminated at the time of
reconstructing are mutually opposite directions, deterioration of
the reconstruction light can be prevented even if a quarter-wave
plate is not used at the recording medium, which is different than
the method disclosed in Optics Letters Vol. 30 p 878-880 (2005);
(3) recording using a rotating-type medium is possible without the
need to use a quarter-wave plate at the recording medium; and the
like.
Note that, in the above description, explanation is given of an
example in which the focal point positions of the two lenses are
made to coincide with one another, and the hologram recording
medium is disposed at a position which is offset in the optical
axis direction from the focal point positions (the FT plane).
However, the focal point positions of the two lenses may be made to
be different.
As shown in FIG. 3, given that the focal length of the lens 34 is
f.sub.1 and the focal length of the lens 48 is f.sub.2, in a case
in which the distance between the lens 34 and the lens 48 is
smaller than the sum (f.sub.1+f.sub.2) of their focal lengths, the
collecting plane of the signal light and the collecting plane of
the reference light are different. At this time, in the collecting
plane of the signal light, the signal light (shown by the dashed
line) and the reference light (shown by the solid line) are
superposed over a wide region. Accordingly, the hologram recording
medium 38 is disposed at the collecting plane of the signal light,
and recording of a hologram can be carried out.
At the collecting plane, the signal light is collected at the
smallest region, and therefore, the recording region can be made to
be small. As a result, a high recording density can be achieved.
Moreover, because the signal light and the reference light are
superposed over a wide region in the optical axis direction, even
if a thick medium is used, the hologram can be recorded over a wide
region in the direction of the film thickness (the optical axis
direction). Therefore, the Bragg condition becomes stricter, and
even higher recording density can be achieved.
However, when the hologram recording medium is disposed at the
collecting plane of the signal light, there are cases in which the
dynamic range of the recording medium suffers a loss due to the
high-intensity zeroth-order light. In such a case, it is preferable
to place the recording medium at a position which does not include
the two collecting planes of the signal light and the reference
light.
Note that, even in cases in which the distance between the lens 34
and the lens 48 is greater than the sum (f.sub.1+f.sub.2) of the
respective focal lengths, the signal light and the reference light
are superposed over a wide region, at the collecting plane of the
signal light, and therefore, effects which are similar to those
described above can be achieved. Further, because the distance
between the lens 34 and the lens 48 is not equal to the sum
(f.sub.1+f.sub.2) of the respective focal lengths, as shown by the
dashed line, the reconstruction light from the hologram recording
medium 38 is not collimated at the lens 48, but can be made into
collimated light by the lenses 50, 52 which are disposed at the
light incident side of the detector 54.
Second Embodiment
FIG. 4 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a second embodiment of
the present invention. As shown in FIG. 4, a laser oscillator 60 is
provided at the hologram recording/reconstructing device. Laser
light (P-polarized light) is oscillated and illuminated from the
laser oscillator 60. A shutter 62, which is for blocking laser
light, is disposed at the laser light illuminating side of the
laser oscillator 60, so as to be able to withdraw from the optical
path. A polarization beam splitter 64, which transmits P-polarized
light and reflects S-polarized light, is disposed at the light
transmitting side of the shutter 62.
A quarter-wave plate 78, which converts linearly polarized light
into circularly polarized light or circularly polarized light into
linearly polarized light, is disposed at the light transmitting
side of the polarization beam splitter 64 so as to be able to
withdraw from the optical path. A lens 66, which collects laser
light for reference light and generates reference light formed from
a spherical reference wave, is disposed at the light transmitting
side of the quarter-wave plate 78. The lens 66 irradiates
P-polarized light as reference light onto the hologram recording
medium 68.
It is preferable to use a lens of a small diameter and a short
focal length as the lens 66. By using a lens having a short focal
length, the small-diameter beam emitted from the laser oscillator
60 can be used as is without inserting an enlarging lens or the
like.
A portion of the aforementioned reference light is transmitted
through the hologram recording medium 68. A lens 70 which
collimates the laser light which has passed through the hologram
recording medium 68, a polarizing plate 72 through which
P-polarized light is transmitted and which blocks S-polarized
light, and a reflection-type spatial light modulator 74 which
modulates laser light for signal light in accordance with a
supplied recording signal of each page and generates the signal
light (P-polarized light) for recording each page of the hologram,
are provided at the light transmitting side of the hologram
recording medium 68. Note that an intensity modulating element,
such as an ND filter or the like, may be disposed between the lens
70 and the polarizing plate 72 in order to adjust the intensity
ratio of the signal light and the reference light.
Lenses 80, 82, and a detector 84, which is structured by an image
pickup element such as a CCD or the like and which converts
received reconstruction light into an electric signal and outputs
the electric signal, are disposed at the light reflecting side of
the polarization beam splitter 64 at the reconstruction light
reconstructed from the recording medium 68. The detector 84 is
connected to a personal computer (not shown). The personal computer
is connected to the spatial light modulator 74 via a pattern
generator. Further, a driving device (not shown), which drives the
shutter 62 and the quarter-wave plate 78 individually, is connected
to the personal computer.
At the time of recording a hologram, first, the driving device (not
shown) is driven, and the shutter 62 and the quarter-wave plate 78
are respectively withdrawn from the optical path such that the
laser light can pass through. Next, the laser light is illuminated
from the laser oscillator 60, a recording signal for each page is
supplied from the personal computer (not shown) to the spatial
light modulator 74 at a predetermined timing, and the hologram
recording processing onto the hologram recording medium 68 is
carried out.
Namely, the laser light (P-polarized light) emitted from the laser
oscillator 60 is incident on the polarization beam splitter 64. The
P-polarized light, which is transmitted through the polarization
beam splitter 64, is collected at the lens 66, and reference light
formed from a spherical reference wave is generated. The generated
reference light of P-polarized light is illuminated onto the
hologram recording medium 68. The laser light which is transmitted
through the hologram recording medium 68 is collimated into a
large-diameter beam at the lens 70, is transmitted through the
polarizing plate 72, and is incident on the spatial light modulator
74 as laser light for signal light.
The incident laser light is modulated by the reflection-type
spatial light modulator 74 in accordance with the supplied
recording signal for each page, and signal light is generated.
Among the light which is modulated at the reflection-type spatial
light modulator 74, only the signal light of P-polarized light
passes through the polarizing plate 72, is collected at the lens
70, and is illuminated onto the hologram recording medium 68 on the
same axis as and from a side different than the reference light.
Due to the signal light and the reference light being illuminated
simultaneously onto the hologram recording medium 68 in this way,
hologram recording of each page is carried out.
When the hologram is reconstructed, first, the driving device (not
shown) is driven, and the quarter-wave plate 78 is inserted onto
the optical path. In this way, the P-polarized light transmitted
through the polarization beam splitter 64 is converted into
circularly polarized light at the quarter-wave plate 78, is
collected at the lens 66, and is illuminated onto the hologram
recording medium 68. The reconstruction light diffracted at the
hologram recording medium 68 becomes reverse-direction circularly
polarized light, and emerges at the lens 66 side.
The reverse-direction circularly polarized light is transmitted
through the lens 66, is converted into linearly polarized light at
the quarter-wave plate 78, and is incident on the polarization beam
splitter 64. Only the S-polarized light which is the reconstruction
light is selectively reflected at the polarization beam splitter 64
and is received at the detector 84 via the lenses 80, 82, and the
hologram image of each page is reconstructed.
As described above, in the second embodiment, in the same way as in
the first embodiment, because the signal light and the reference
light are illuminated onto the hologram recording medium on the
same axis and from opposite directions, a reflection-type hologram
can be recorded. In this reflection-type hologram, a hologram can
be multiple-recorded by utilizing the direction of thickness of the
recording medium, and high density recording can be achieved.
Further, in the second embodiment, because the reference light,
which is transmitted through the hologram recording medium, is used
as the light for the signal light, the loss of the light amount can
be decreased. Further, the optical system for making the signal
light and the reference light coaxial is simple, the device can be
made more compact and lower-cost, and the failure rate can be
reduced.
Note that, in a case in which shift multiple recording is carried
out, other than carrying out the multiple recording while rotating
the hologram recording medium 68 around an axis of rotation which
is parallel to the optical axis, as shown in FIG. 5, multiple
recording may be carried out while moving the hologram recording
medium 68 parallel to a direction along the main surface thereof
(the direction of arrow A). Or, multiple recording may be carried
out while rotating the hologram recording medium 68 in the
direction of arrow B so that the optical axis and a normal line of
the main surface of the hologram recording medium 68 intersect one
another.
Further, in the structure shown in FIG. 4, in a case in which a
short focal length lens is used as the lens 66, the lens 66 and the
polarization beam splitter 64 are too close to one another, and it
is physically difficult to place the quarter-wave plate 78 between
the lens 66 and the polarization beam splitter 64. In this case, as
shown in FIG. 6, it is preferable to insert a long focal length
lens 86 between the lens 66 and the polarization beam splitter
64.
When the collimated light, which has passed through the
polarization beam splitter 64, is transmitted through the long
focal length lens 86, the transmitted light is no longer strictly
collimated light, but because the focal length of the lens 86 is
long, it can be considered to be substantially collimated light.
Accordingly, given that the focal length of the lens 66 is f.sub.3
and the focal length of the lens 70 is f.sub.4, by making the
interval between the lens 66 and the lens 70 be (f.sub.3+f.sub.4),
the light which is transmitted through the lens 70 can be made to
be substantially collimated light.
Further, in order to generate even more strict collimated light by
the lens 70, it is preferable that the interval between the lens 66
and the lens 70 be made to be smaller than (f.sub.3+f.sub.4). At
the time of reconstructing, the reconstruction light which is
generated from the recording medium 68 is Fourier transformed by
the lens 66 and is imaged between the lens 66 and the lens 86, and
that image is relayed by the lens 86 and the lens 82 and is
incident on the detector 84. By inserting the long focal length
lens 86, the distance between the lens 66 and the lens 86 can be
made to be large, and it is easy to ensure a place for placement of
the quarter-wave plate 78.
Moreover, as shown in FIG. 7, a diffraction element 76, such as a
computer generated hologram (CGH) or the like, can be inserted
between the laser oscillator 60 and the polarization beam splitter
64. Other than inserting the diffraction element 76, the structure
is the same as that of the device shown in FIG. 4. By inserting the
diffraction element 76, the hologram recording medium 68 is placed
at the Fourier transform plane of the signal light, and a hologram
can be recorded.
Third Embodiment
FIG. 8 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a third embodiment of
the present invention. As shown in FIG. 8, in this hologram
recording/reconstructing device, a diffraction element 76A, which
modulates the intensity distribution or the phase distribution of
the incident light, and lenses 120, 122 are inserted between the
laser oscillator 60 and the polarization beam splitter 64. Further,
a light-shielding plate 124, in which an aperture 124A is formed,
is disposed next to the hologram recording medium 68, at the signal
light incident side of the hologram recording medium 68. Other than
these points, the structure is the same as that of the hologram
recording/reconstructing device shown in FIG. 4. Therefore, the
same reference numerals are applied to the same structural
portions, and description thereof will be omitted.
A computer generated hologram (CGH) or the like, which can design
the pattern of the reference light at the Fourier transform plane
which is the position of the hologram recording medium 68, can be
used as the diffraction element 76A. A phase-modulating-type
diffraction element is preferable as the diffraction element 76A.
By using kinoform in particular, the loss of the amount of light
can be reduced.
The lens 120 and the lens 122 enlarge or reduce, at a magnification
m, the light obtained by the emitted light of the laser oscillator
60 being modulated by the diffraction element 76A, and relay it to
a front focal plane of the lens 66. As will be described later, the
relayed image is Fourier transformed by the lens 66, is illuminated
onto the recording medium 68, interferes with the signal light, and
records a hologram.
Here, the efficiency of utilization of the light can be improved by
designing the pattern of the reference light such that only a
specific frequency region of the signal light is illuminated at the
reference light. For example, given that the pixel interval of the
spatial light modulator 74 is p.sub.2 and the focal length of the
lens 70 is f.sub.4, all of the spatial frequency components
included in the signal light transmitted through the aperture 124A
can be assumed to be within a square range which is in the Fourier
transform plane and whose center is the optical axis and whose
length of a side is less than or equal to f.sub.4/p.sub.2. In this
case, if the maximum spatial frequency component included in the
reference light is in a wider range which encompasses the
aforementioned range, the signal light and the reference light are
superposed at the Fourier transform plane, and a hologram can be
recorded. To this end, given that the focal length of the lens 66
is f.sub.3 and the pixel interval of the diffraction element 76A is
q, it is preferable that f.sub.4/p.sub.2.ltoreq.f.sub.3/(mq), and
more preferable that f.sub.4/p.sub.2=f.sub.3/(mq). Note that m is
the enlargement magnification of the lens 120 and the lens 122.
Given that the focal length of the lens 70 is f.sub.4, the pixel
interval of the spatial light modulator 74 is p.sub.2, and the
wavelength of the laser light is .lamda., the size of the aperture
124A is preferably within the range of 1.lamda.f.sub.4/p.sub.2 to
2.5.lamda.f.sub.4/p.sub.2, and is more preferably within the range
of 1.lamda.f.sub.4/p.sub.2 to 1.5.lamda.f.sub.4/p.sub.2.
The structure of the diffraction element 76A used in the present
embodiment is shown in FIGS. 9A and 9B. This flat-plate-shaped
diffraction element 76A is structured by a hollow region 128 which
is punched-out in the form of a rectangle, and an outer frame
region 126 surrounding the periphery thereof. The intensity
distribution or the phase distribution of the incident light is
modulated at the outer frame region 126, but is not modulated at
the hollow region 128. Here, the rectangular hollow region 128 is a
shape similar to the signal light pattern region at the
reflection-type spatial light modulator 74. By designing the
surface area of the hollow region 128 at the diffraction element
76A, it is possible to illuminate only collimated light on the
reflection-type spatial light modulator 74. Signal light of good
quality can thereby be obtained. Given that the surface area of the
hollow region 128 is A, the magnification of the surface area
enlarged by the lens 66 and the lens 70 is M, and the surface area
of the region of the signal light pattern of the reflection-type
spatial light modulator 74 is B, it is preferable that
A.gtoreq.B/M, and more preferable that A.gtoreq.1.2B/M. Another
advantage resulting from designing the surface area A of the hollow
region 128 is that the intensity balance of the signal light and
the reference light can be adjusted.
At the time of recording a hologram, first, the driving device (not
shown) is driven, and the shutter 62 and the quarter-wave plate 78
are respectively withdrawn from the optical path such that the
laser light can pass through. Next, the laser light is illuminated
from the laser oscillator 60, a recording signal for each page is
supplied from the personal computer (not shown) to the spatial
light modulator 74 at a predetermined timing, and the hologram
recording processing onto the hologram recording medium 68 is
carried out.
The laser light emitted from the laser oscillator 60 is incident on
the diffraction element 76A. The incident laser light passes
through the hollow region 128 of the diffraction element 76A as
shown by the dashed line in FIG. 10. However, at the outer frame
region 126, as shown by the solid line, the intensity or the phase
is modulated to a predetermined pattern, and the light exits. The
laser light which exits from the diffraction element 76A is
transmitted through the lenses 122, 120, and is incident on the
polarization beam splitter 64.
The P-polarized light transmitted through the polarization beam
splitter 64 is collected at the lens 66, whose front focal plane is
the rear focal plane of the lens 120. Reference light, which is
formed from a diffraction pattern due to the intensity distribution
or the phase distribution of the diffraction element 76A, is
generated. The generated reference light of P-polarized light is
illuminated onto the hologram recording medium 68. The laser light
transmitted through the hologram recording medium 68, and in
particular, the zeroth-order light passing through the hollow
region 128 of the diffraction element 76A, is collimated into a
large-diameter beam at the lens 70, passes through the polarizing
plate 72, and is incident on the spatial light modulator 74 as
laser light for signal light.
The incident laser light is modulated by the reflection-type
spatial light modulator 74 in accordance with the supplied
recording signal for each page, and signal light is generated.
Among the light which is modulated at the reflection-type spatial
light modulator 74, only the signal light of P-polarized light
passes-through the polarizing plate 72, is collected at the lens
70, passes through the aperture 124A formed in the light-shielding
plate 124, and is illuminated onto the hologram recording medium 68
on the same axis as and from a different side than the reference
light. By simultaneously illuminating the signal light and the
reference light onto the hologram recording medium 68 in this way,
hologram recording of each page is carried out.
Because the method of reconstructing the hologram is similar to
that of the second embodiment, description thereof will be
omitted.
As described above, in the third embodiment, in the same way as in
the first and second embodiments, a reflection-type hologram can be
recorded, and high-density recording can be achieved by multiple
recording of a hologram utilizing the direction of thickness of the
recording medium. Moreover, in the same way as in the second
embodiment, because the reference light, which is transmitted
through the hologram recording medium, is used as the light for the
signal light, the loss of the light amount can be decreased.
Further, the optical system for making the signal light and the
reference light coaxial is simple, the device can be made more
compact and lower-cost, and the failure rate can be reduced.
In addition, in the third embodiment, by inserting the diffraction
element, the region where the signal light and the reference light
are superposed at the Fourier transform plane can be optimized, and
therefore, a high light utilization efficiency can be realized. As
a result, the time for recording the hologram can be shortened.
Further, the size of the hollow region 128 or the size of the
spatial light modulator 74 is adjusted, such that only the light
(shown by the dashed line) which passes through the hollow region
128 of the diffraction element 76A is incident on the spatial light
modulator 74 as shown in FIG. 10. In this way, light other than the
light which passes through the hollow region 128 is not incident on
the spatial light modulator 74. Accordingly, low-noise,
high-quality signal light can be generated.
Fourth Embodiment
FIG. 11 is a schematic structural diagram of a hologram
recording/reconstructing device relating to a fourth embodiment of
the present invention. As shown in FIG. 11, a laser oscillator 90
is provided at the hologram recording/reconstructing device. Laser
light (P-polarized light) is oscillated and illuminated from the
laser oscillator 90. A shutter 92, which is for blocking laser
light, is disposed at the laser light illuminating side of the
laser oscillator 90, so as to be able to withdraw from the optical
path. A polarization beam splitter 94, which transmits P-polarized
light and reflects S-polarized light, is disposed at the light
transmitting side of the shutter 92.
A quarter-wave plate 106, which converts linearly polarized light
into circularly polarized light or circularly polarized light into
linearly polarized light, is disposed at the light transmitting
side of the polarization beam splitter 94, so as to be able to
withdraw from the optical path. A lens 96, which collects laser
light for reference light and generates reference light which is
formed from a spherical reference wave, is disposed at the light
transmitting side of the quarter-wave plate 106. The lens 96
illuminates P-polarized light as reference light onto a hologram
recording medium 98. A reflecting mirror 108 is disposed at the
light reflecting side of the polarization beam splitter 94.
The aforementioned reference light passes through the hologram
recording medium 98. A lens 100, which collimates the laser light
which has passed through the hologram recording medium 98, and a
polarization beam splitter 102 are disposed at the light
transmitting side of the hologram recording medium 98. A
quarter-wave plate 110 is disposed between the lens 100 and the
polarization beam splitter 102, so as to be able to withdraw from
the optical path. A reflection-type spatial light modulator 104,
which modulates the laser light for signal light in accordance with
a supplied recording signal of each page and generates signal light
for recording each page of the hologram, is provided at the light
transmitting side of the polarization beam splitter 102.
A detector 112, which is structured by an image pickup element such
as a CCD or the like and which converts received reconstruction
light into an electric signal and outputs the electric signal, is
disposed at the light reflecting side of the polarization beam
splitter 102. The detector 112 is connected to a personal computer
(not shown). The personal computer is connected to the spatial
light modulator 104 via a pattern generator. Further, a driving
device (not shown), which drives the shutter 92 and the
quarter-wave plates 106, 110 individually, is connected to the
personal computer.
At the time of recording a hologram, first, the driving device (not
shown) is driven, and the shutter 92 and the quarter-wave plates
106, 110 are respectively withdrawn from the optical path such that
laser light can pass through. Next, the laser light is illuminated
from the laser oscillator 90, a recording signal for each page is
supplied from the personal computer (not shown) to the spatial
light modulator 104 at predetermined timings, and hologram
recording processing onto the hologram recording medium 98 is
carried out.
Namely, the laser light emitted from the laser oscillator 90 is
incident on the polarization beam splitter 94. The P-polarized
light which passes through the polarization beam splitter 94 is
collected at the lens 96, and reference light formed from a
spherical reference wave is generated. The generated reference
light of P-polarized light is illuminated onto the hologram
recording medium 98. The laser light which is transmitted through
the hologram recording medium 98 is collimated into a
large-diameter beam at the lens 100, passes through the
polarization beam splitter 102, and is incident on the spatial
light modulator 104 as laser light for signal light.
The incident laser light is modulated by the reflection-type
spatial light modulator 104 in accordance with the supplied
recording signal of each page, and signal light is generated. Among
the light which is modulated at the reflection-type spatial light
modulator 104, only the signal light of P-polarized light is
transmitted through the polarization beam splitter 102, is
collected at the lens 100, and is illuminated on the hologram
recording medium 98 on the same axis as and from a side different
than the reference light. Due to the signal light and the reference
light being illuminated onto the hologram recording medium 98
simultaneously in this way, hologram recording of each page is
carried out.
Note that, at the time of recording the hologram, the polarization
beam splitter 94 is not a requisite structural element. Therefore,
the polarization beam splitter 94 may be withdrawn from the optical
path at the time of recording a hologram, and may be inserted onto
the optical path at the time of reconstructing a hologram.
At the time of reconstructing a hologram, first, the driving device
(not shown) is driven, and the quarter-wave plates 106, 110 are
inserted onto the optical path. In this way, the P-polarized light,
which has passed through the polarization beam splitter 94, is
converted into circularly polarized light at the quarter-wave plate
106, is collected at the lens 96, and is illuminated onto the
hologram recording medium 98. The reconstruction light diffracted
at the hologram recording medium 98 becomes reverse-direction
circularly polarized light, and exits at the lens 96 side.
The reverse-direction circularly polarized light passes through the
lens 96, is converted into linearly polarized light (S-polarized
light) at the quarter-wave plate 106, and is incident on the
polarization beam splitter 94. Only the S-polarized light which is
the reconstruction light is selectively reflected at the
polarization beam splitter 94 and is emergent. The light emerging
from the polarization beam splitter 94 is reflected at the mirror
108, and is again incident on the polarization beam splitter 94.
The polarized light of the reflected light from the mirror 108 is
still S-polarized light. The reflected light of this S-polarized
light is reflected at the polarization beam splitter 94 and
exits.
The S-polarized light, which again exits from the polarization beam
splitter 94, is converted into reverse-direction circularly
polarized light at the quarter-wave plate 106, is collected at the
lens 96, and is illuminated onto the hologram recording medium 98.
The reconstruction light which is transmitted through the hologram
recording medium 98 is collimated by the lens 100, is converted
into linearly polarized light (S-polarized light) by the
quarter-wave plate 110, and is incident on the polarization beam
splitter 102. Only the S-polarized light which is the
reconstruction light is selectively reflected at the polarization
beam splitter 102 and is emergent. The emergent light is received
at the detector 112, and the hologram image of each page is
reconstructed.
As described above, in the fourth embodiment, in the same way as in
the first and second embodiments, a reflection-type hologram can be
recorded, and high-density recording can be achieved by multiple
recording of a hologram utilizing the direction of thickness of the
recording medium. Moreover, in the same way as in the second
embodiment, because the reference light, which is transmitted
through the hologram recording medium, is used as the light for the
signal light, the loss of the light amount can be decreased.
Further, the optical system for making the signal light and the
reference light coaxial is simple, the device can be made more
compact and lower-cost, and the failure rate can be reduced.
In addition, in the fourth embodiment, due to phase conjugation,
the effects of strain, which is generated at each optical element,
on the reconstruction light can be made to be small. Therefore, the
bit error rate can be made to be small.
In the above explanation, the polarization beam splitter 102 is
disposed in a vicinity of the reflection-type spatial light
modulator 104. However, in a case in which a polarization hologram
is recorded in and reconstructed from a medium at which polarized
light recording is possible, the polarization beam splitter 102 can
be omitted. In this case, at the time of reconstructing, the
detector 112 is disposed in place of the reflection-type spatial
light modulator 104. There is also no need to insert the
quarter-wave plates 106, 110.
In the above second and third embodiments, description is given of
a method of recording and reconstructing a hologram in accordance
with intensity modulation. A case of using a medium at which
polarized light recording is possible in the second in third
embodiments will be described hereinafter. For example, in FIG. 4
or FIG. 8, in the case of using a medium at which polarized light
recording is possible as the hologram recording medium 68, the
transmission axis is set such that the polarizing plate 72
transmits S-polarized light and blocks P-polarized light. In this
way, a polarization hologram can be recorded in the hologram
recording medium 68 by signal light of S-polarized light and
reference light of P-polarized light.
At the time of reconstructing, there is no need to insert the
quarter-wave plate 78. When the P-polarized light of the reading
light is illuminated onto the polarization hologram recorded in the
hologram recording medium 68, the reconstruction light of
S-polarized light exits in the direction of the polarization beam
splitter 64, is reflected by the polarization beam splitter 64, and
is detected at the detector 84. In this case, the noise of the
P-polarized light which is caused by the scattering of the reading
light is transmitted through the polarization beam splitter 64 and
is not incident on the detector 84, because the plane of
polarization thereof is orthogonal to that of the reconstruction
light of S-polarized light. Accordingly, the SN ratio of the
reconstructed image detected at the detector 84 can be made to be
large.
Adjustment of the intensity ratio of the signal light and the
reference light can also be carried out by disposing an intensity
modulating element such as an ND filter at an arbitrary position
between the hologram recording medium 68 and the spatial light
modulator 74. The intensity modulating element is preferably
disposed between the polarizing plate 72 and the spatial light
modulator 74. Note that, in the fourth embodiment as well,
adjustment of the intensity ratio of the signal light and the
reference light can be carried out by disposing an intensity
modulating element such as an ND filter between the polarization
beam splitter 102 and the spatial light modulator 104.
* * * * *